shouldn't it make sense for banks to at least put forth pretty clear warnings on money that has not really been validated yet? Or to at least proactively warn anyone seeking to withdraw money that hasn't really been validated that if the check fails to validate, they may be liable?

I think the main point is that only a minimal amount of effort is required to remind people that just because a check has cleared, it doesn't mean the funds are guaranteed. Maybe require that all depositors be issued a warning if they make a deposit above a certain amount (say $10,000).